In general, the present invention is directed to the protection of digital media, of printed documents, and of physical objects by means of modification of content or object features, such modification being supposed to enable the production of robust and secure fingerprints. The obtained fingerprints are, amongst many possible applications, mainly used for the content and/or object authentication, identification, or detection of local modification.
To present, there exist in general two methods allowing to realize such kind of protection which, however, use fundamentally different approaches, namely on the one side conventional content fingerprinting, also known as robust hashing, and on the other side digital watermarking, also referred to as data hiding.
Conventional content fingerprinting or robust hashing are techniques which extract a fingerprint directly from the object features and no preliminary content pre-processing is performed. As a result, the fingerprints are not very robust. Such lack of robustness leads to a large amount of errors or mismatches between the original fingerprint and the fingerprint extracted from a distorted counterpart of the original object. In turn, this results in most applications in the necessity to perform a highly complex search and identification for trying to match the original fingerprint and the extracted fingerprint. In authentication applications, this leads to higher privacy leaks due to the necessity to store large amounts of helper data which is somewhat proportional to the level of mismatch. Therefore, reducing the level of mismatch is of great importance in conventional content fingerprinting.
Digital watermarking or data hiding realize content modification by embedding a special mark or encoded message which carries information on the protected content. The content should be modified in such a way to ensure reliable and secure extraction of an embedded, desired message. This has two important consequences concerning the security as well as the cancellation of interference between content and embedded message. With respect to security, the need to embed the encoded message requires the use of special error correction codes that introduce redundancy which, in turn, can be efficiently used by attackers to attack the content by message removal key learning, thus reducing the level of security. With respect to cancellation of interference between the content and the embedded message, the content modification should, of course, be performed in such a way that the content features do not interfere with the embedded message, which in turn requires special techniques of modulation that by design are not secure and not robust with respect to several groups of distortions used by attackers trying to decode or remove the watermark. Therefore, whilst not requiring a highly complex search and identification at the time of extraction, watermarking bears inherent disadvantages due to the type of content modification performed.
In short, the above mentioned existing procedures inherently comprise several problems with respect to performance or security, since these approaches either require performing complex search and identification or introducing redundancy, respectively insecure modulation techniques.